1. Field of the Invention
The present invention relates to constant volume (CV) combustion chambers and combustors and methods for their operation.
2. Description of Related Art
Constant volume combustion is fundamentally different and operates on a different principle from constant pressure combustion. A constant pressure combustor uses an open combustion chamber and high pressure is maintained in the combustion chamber during combustion by supplying high-pressure fuel and oxidizer to the combustion chamber.
A pulse detonation combustor uses an open combustion chamber and mimics true constant volume combustion by combusting fuel so quickly that combustion is complete before combustion product exhaust exits the open combustion chamber. Fuel and oxidizer are injected into the combustion chamber in pulses, resulting in sequential detonations that create pressure pulses of exhaust from a nozzle. Unlike constant pressure or pulse detonation rocket motors, a constant volume engine employs an adiabatic isochoric deflagration process to achieve combustion of the fuel-oxidizer mixture. The combustion chamber is sealed during combustion and opened at an optimal time after combustion initiation to release combustion products through a nozzle to provide thrust.
Constant pressure combustors may experience reduced efficiency as ambient pressure changes, for example with altitude. Plugs or pintles have been used in combination with constant pressure combustors to overcome this limitation. For example, U.S. Pat. No. 5,394,690 discloses a pressure-controlled pintle to provide a constant pressure and variable thrust from an engine. In response to the pressure in the combustion chamber falling below a design pressure, the pintle closes a gap between the pintle and a nozzle throat to lower the area of the throat to re-establish design pressure in the chamber. U.S. Pat. No. 6,591,603 B2 discloses a rocket engine that is self-compensating on nozzle thrust coefficient for varying ambient backpressures. The rocket engine includes a nozzle plug that aerodynamically self-compensates for changes in ambient backpressure at the nozzle exit cone such that the nozzle thrust coefficient is maximized for any ambient backpressure. U.S. Pat. No. 7,656,979 B2 discloses varying the direction and magnitude of thrust from a solid or liquid fuel rocket motor by changing the relative positions of a plug and a combustion chamber outlet. The plug is moved to maintain efficient operation of the motor, for example, at different altitudes.
In these cases, the pintle or plug must never seal the combustion chamber because the fuel and oxidizer enter the combustion chamber at a near steady rate and blocking the exhaust during normal operation would result in an explosion.
Pulse detonation combustors suffer from drawbacks associated with structural requirements to prevent failure and the complexity of fuel delivery and ignition systems required to obtain properly timed detonations. One way of overcoming some of these drawbacks is described in U.S. Pat. No. 7,047,724 B2, which discloses a combustor comprising multiple, distributed ignition sources to obtain constant volume-like combustion performance in a pulse combustion device in the absence of detonation. The improvement mitigates some of the drawbacks associated with detonation-type combustion and resulting shock waves, but the ignition system of such a combustor remains relatively complex in order to obtain complete combustion during each pulse before combustion products escape the open end of the combustion chamber.
Advantages of the constant volume combustion cycle relative to pulse detonation combustors include simpler ignition systems and a reduction in the extremely high pressures. Advantages of the constant volume combustion cycle over conventional liquid bipropellant constant-pressure combustion cycles include the possible elimination of high-pressure pumps and improved Isp for a given system supply pressure. U.S. Pat. No. 7,631,487 B2, which is incorporated by reference in its entirety, discloses a constant volume rocket motor with a combustion chamber comprising a reciprocating thrust valve that controls the flow of combustion products through an outlet nozzle. The rocket motor comprises a combustor capable of producing pressure pulses of between 500 and 700 PSI. The reciprocating thrust valve seals against a nozzle seat on a wall of a combustion chamber. Wear of the thrust valve and the throat of the combustion chamber outlet is mitigated by a canted-coil on the pintle. A simple pintle without a canted coil is disclosed in the '487 patent in an example as being “non-optimal” because of expected wear that would prevent adequate sealing of the chamber.
The combustor of the constant volume rocket motor disclosed in the '487 patent is limited with respect to the need for a canted coil pintle to prevent wear on surfaces used to seal the combustion chamber and effectively sealing the combustion chamber during combustion at very high pressures, such as pressures above 700 PSI.